1. Field of the Invention
This invention is related generally to a method for inducing span-wise twist in hollow beams such as airfoils or rotor blades using a shear warp actuator that is integrated into the beam along its span.
2. Related Technology
Most airfoil blades, once made, cannot change their twist (relative cross-section rotational) along the blade span. Many non-rotating blade systems (e.g., airplane wings and control surfaces) have no built-in twist, whereas many of the rotating airfoil blade systems (e.g., helicopter rotor blades) have some fixed amount of twist built into them. Recent research and development has been focused on active twist control of helicopter rotor blade systems primarily for mitigation of aerodynamically-induced vibrations, as discussed in Chopra, I., “Status of Application of Smart Structures Technology to Rotorcraft Systems”, J. Amer. Helicopter Soc., Vol. 45, No. 4, 2000, pp. 228-252, and Giurgiutiu, V. “Recent Advances in Smart-Material Rotor Control Actuation”, AIAA Paper#2000-1709, in Proc. 41st AIAA/ASME/ASCE.AHS/ASC Structures, Structural Dynamics, and Materials Conference, Atlanta, Ga.
Some developments in active control of rotor blade twist are described in: Chen, P., and Chopra, I., “Hover Testing of Smart Rotor with Induced-Strain Actuation of Blade Twist”, AIAA Journal, Vol. 35, 1997; Wilbur, M. et al., “Hover Testing of the NASA/Army/MIT Active Twist Rotor Prototype Blade”, AHS 56th Annual Forum, May 2000; Bothwell, C. et al., “Torsional Actuation with Extension-Torsion Composite Coupling and a Magnetostrictive Actuator”, AIAA Journal, Vol. 33, 1995; Derham, R. et al., “Design Evolution of an Active Materials Rotor”, in Proc. AHS 57th Annual Forum, May 2001; and Jacot et al., U.S. Pat. No. 6,065,934, entitled “Shape Memory Rotary Actuator”. U.S. Pat. No. 6,970,773 to Phillips discloses a system for inducing an optimized twist distribution for a wing. U.S. Pat. No. 5,681,014 to Palmer discloses a torque tube-based system for twisting an airfoil. U.S. Pat. No. 5,505,589 to Bergey discloses a controllable variable twist rotor blade assembly for rotary wing aircraft. U.S. Pat. No. 6,024,325 to Carter, Jr. discloses a coil-spring system for controlling pitch of a rotor for a rotary wing aircraft. U.S. Pat. No. 6,065,934 to Jacot et al. discloses a shape memory rotary actuator for a rotor blade. U.S. Pat. No. 6,497,385 to Wachpress et al. discloses a rotor blade with optimized twist distribution.
These active twist systems typically use piezoelectric or magnetostrictive actuators embedded in the composite structure of closed-section rotor blades. The large torsional rigidity of the closed cross-section blades requires large actuation forces to achieve a given degree of twist. Active-twist designs with structure-embedded actuation have so-far been limited to a few degrees of twist or less over the length of the blade. Deformation of closed-section beams by embedded or external actuators requires large amounts of actuation force and energy because of the large elastic stiffness and strain energy associated with the deforming member under twist.